introduction to-plant_pathology
TRANSCRIPT
Introduction to Plant Introduction to Plant PathologyPathology
AND environmental impact
Disease = disturbance from plant Disease = disturbance from plant pathogen or environmental factor that pathogen or environmental factor that interferes with plant physiologyinterferes with plant physiologyCauses changes in plant appearance or
yield lossDisease results from:Direct damage to cellsToxins, growth regulators, or other
byproducts that affect metabolismUse of nutrients and water or interference
with their uptake
Disease PyramidDisease Pyramid
The interaction of components of plant disease can be expanded to include time and humans.
Time is often considered as the fourth component of plant disease development.
The four components together can quantify the amount of disease.
The human equation can affect the three components of the disease triangle and should be considered as a fifth component in disease development.
Host FactorsHost Factors
All plants can be considered hostsDegree of genetic uniformity – crop plants – inbred
linesAge – affects disease development depending on
plant-pathogen interactionThere are different levels of susceptibility, which
include:◦ Immune - cannot be infected. ◦ Susceptible - can be infected. ◦ Resistant - may or may not be infected, and is the plant able
to prevent the pathogen from killing it. ie. defense compounds
Pathogen FactorsPathogen Factors
Amount of inoculumPathogen geneticsVirulence of the pathogenType of reproduction:
◦Monocyclic◦polycyclic
Ecology and mode of spread◦Air◦Soil◦Seed◦Vector dependency
Environmental FactorsEnvironmental Factors
MoistureTemperatureEffect of human culture practice
◦Monoculture◦Amount of inoculum: seed quality, disease residues, rotation, alternate host
◦Introduction of new pathogens
Disease DevelopmentDisease Development
Every infectious disease requires a series of sequential events in order for disease to develop.
Specific characteristics are unique for each disease.
General events are:1. dispersal of the pathogen to the host2. penetration and infection of the host3. invasion and colonization of the host4. reproduction of the pathogen5. pathogen dispersal6. pathogen survival between growing seasons
and/or in the absence of a host
FungiFungi
Diverse and widespreadFilamentous (hyphae) form a network of
mycelium (lots of hyphae)Recognized by reproductive structures
(mushrooms, rusts, conks, etc.)Most of the 100,000 spp. are saprophytes
◦Live on dead organic matterApproximately 8,000 species attack plants
◦Plant pathogens
Fungal DiseasesFungal Diseases
Reproduction by sexual and asexual means
Spread through a variety of methods◦ wind/water blown spores◦ rhizomorphs◦ Sclerotia (overwintering)
Include organisms from Kingdom Protista, that are now classified outside the Kingdom Fungi:◦ Downy mildews◦ Pythium◦ Phytophthora◦ Clubroots
SymptomsSymptoms
Initially, similar to drought & starvation:◦Plants appear off-colour◦Weakened & susceptible to attack◦Wilting and dieback occur later◦Younger plants usually killed rapidly◦Older plants decline over time (years)◦Roots have brownish streaks
BacteriaBacteria
Prokaryotic microscopic organisms◦ Free living single cells, or◦ Filamentous colonies
Reproduce via binary fission◦ 2 daughter cells are identical to mother cell
Don’t usually produce resistant resting spores◦ Need host or growth medium to survive
For rapid spread, plant infecting bacteria usually require:
◦ Warmth◦ Moist conditions
Bacterial DiseasesBacterial Diseases
Less common than fungal or viral diseasesThey can be either:
◦parasites, saprophytes, or autotrophsSymptoms include:
◦Cankers, Wilts, Shoot Blights, Leaf Spots,Scabs, Soft Rots, & Galls
Generally, cannot invade healthy tissue; need wound or opening to infect.
Control methods usually cultural in nature (don’t use antibiotics on large scale)
Bacterial DiseasesBacterial Diseases
Bacterial galls: In some cases, toxic materials are produced that cause plant tissues of roots, stems or leaves to grow abnormally as in crown gall.
Bacterial leaf spot disease: The bacteria usually enter through leaf stomata. Symptoms include water-soaking, slimy texture, fishy or rotten odor, confined initially between leaf veins resulting in discrete spots that have straight sides and appear angular.
Gene on gene action!Gene on gene action!
Evolution of the plant–bacterial pathogen interaction. (a) Plants have evolved receptors that could recognize PAMPs and triggers basal defence. (b) Bacterium injects effector protein through type III secretion system (T3SS) to interfere with defence signalling or response. (c) Plant responds to infection by generation of immune receptors encoding for nucleotide-binding (NB), MAP kinase, leucine-rich-repeat (LRR) R-proteins that recognizes effector protein and triggers an acute defence response usually involving hypersensitive response (HR) and programmed cell death
Disease DevelopmentDisease Development
Infections occur through leaf scars and wounds. These give rise to small cankers in which the bacteria survive the winter.
Rain or water splash, and pruning tools spread the bacterium.
Bacteria overwinter in active cankers, in infected buds, and on the surface of infected and healthy trees and weeds.
The bacterium reproduces best between 21ºC and 25ºC.
The disease seems to be more severe after cold winters and prolonged spring rains.
VirusesViruses
Viruses are "submicroscopic" entities that infect
individual host plant cells. Viruses are obligate parasites: They can only
replicate themselves within a host's cell. In the virus infected plant, production of
chlorophyll may cease (chlorosis, necrosis)Cells may either grow and divide rapidly or may
grow very slowly and be unable to divide
Viral DiseasesViral Diseases
> 400 viruses infect plants; few are economically important pathogens
The infection remains foreverViruses are transmitted from plant to plant
by living factors: insects, mites, fungi and nematodes
Or non-living factors: rubbing, abrasion or other mechanical means (including grafting or other forms of vegetative propagation)
Occasionally transmitted in seed.
Virus Disease SymptomsVirus Disease Symptoms
The symptoms of most virus diseases can be put into four categories:
1. Lack of chlorophyll formation in normally green organs
2. Stunting or other growth inhibition 3. Distortions 4. Necrotic areas or lesions
NematodesNematodesMicroscopic roundworms◦ Barely visible with naked eye◦ No segments
Up to 4mm longClear or transparentFeed with stylet
◦ Pierce plants (pests)◦ Kill arthropods (beneficials)
Nematode DiseasesNematode Diseases
Plant pathogenic nematodes = pests◦ Infect roots & bulbs (below-ground)
◦ Foliar nematodes (above-ground)
◦ Also vectors of plant viruses
As they feed, they weaken & stress plants – also predispose to other problems
Causes bulb & root decline, and root knotsSpread by splashing water, and infested soil &
plant parts
Shoot NematodesShoot Nematodes ((AphelenchoidesAphelenchoides spp.) spp.)
Foliar nematodes feed inside leaves between
major veins causing chlorosis and necrosis.
Injury is most often seen at the base of older
foliage.
When plants with a net-like pattern of veins
become infested with foliar nematodes, the
tissues collapse in wedge-shaped areas and then
change color.
Root NematodesRoot Nematodes
Moisture and nutrient stress symptoms and general stunting are common (by killing meristem tissue)
Root lesion nematodes (Pratylenchus spp.)Burrowing nematodes (Radopholus similis)
destroy root cortex tissues as they feedRoot-knot nematodes (Meloidogyne spp.)
inject growth-regulating substances into root tissues as they feed, stimulating growths called galls or knots
Environmental and cultural Environmental and cultural factors affecting buildup of factors affecting buildup of fungal and bacterial plant fungal and bacterial plant pathogenspathogens
MoistureTemperatureDispersal agentsSoil pHOther
MoistureMoisture
Activates resting stagesAffects germination of spores and
penetration into hostWater on leavesHumiditySplashing water distributes inoculumLeaf wetness = best indicator but
difficult to measure
MoistureMoisture
Activates resting stagesAffects germination of spores and penetration
into hostWater on leavesHumiditySplashing water distributes inoculumLeaf wetness = best indicator but difficult to
measure
Rainy, cloudy conditions = important for spread and growth of many diseases
TemperatureTemperature
Affects growth ratesSome pathogens adapted to certain temp.
rangesRefrigeration = important for
management
Dispersal AgentsDispersal Agents
Bacteria, fungi are limited in mobility, need to be moved by:
WaterWindPeople, machineryInsects, other animals
Soil pHSoil pH
specific requirements for many soil-borne pathogens
Other
Widespread planting of genetically homogeneous crops can favor epidemic
Management of Plant Management of Plant Disease –Disease –StrategiesStrategies
Eliminate or reduce initial inoculum, or delay its introduction (preventive)
Slow the rate of increase, shorten exposure to favorable conditions
Management of Plant DiseaseManagement of Plant Disease
SanitationFungicidesHost plant resistanceCrop rotationCultural practicesTemperatureBiological controlOrganic amendmentsImproved plant health and nutrition
Sanitation (aimed at excluding Sanitation (aimed at excluding pest)pest)
Avoid infested sitesClean soil, planting material, tools, etc.Inspection and quarantineRemove infected debrisTissue culture can provide disease-free
planting material
FungicidesFungicides
Bactericides, if target is bacteriaDusts, sprays, fumigants, etc. Foliar, soil, seed, wound, or post-plant
applicationPreventative – slows rate of increaseInsecticides may also be useful for
managing insect vectors
Host Plant ResistanceHost Plant Resistance
Caution: pathogens can have multiple isolates
Vertical resistance – against some genotypes of a pathogen
Horizontal resistance – not limited to certain genotypes, across all isolates
Host genetic diversity is important to slow epidemics
Crop RotationCrop Rotation
Useful vs soil-borne diseases
Residues of some plants (e.g., cabbage family) may be toxic to some pathogens
Cultural Practices to Cultural Practices to Minimize Spread of DiseaseMinimize Spread of Disease
Favorable irrigation practices (drip vs overhead)
Timing of PlantingWider row spacingsEradicate alternate hosts for viruses
Important to minimize water and humidity to limit disease spread
Moisture management
TemperatureTemperature
Heat for soil sterilizationHot water treatment of planting materialSolarizationRefrigeration to slow disease progress in
harvested material
Management of Plant DiseaseManagement of Plant Disease
Sanitation Fungicides Host plant resistance Crop rotation Cultural practices Temperature
Biological control – Rhizobacteria may interfere with colonization of plant roots by fungi and bacteria
Organic amendments (avoid diseased plants in mulch, etc.)
Improved plant health and nutrition
The End!The End!
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